The present invention relates to endless belts having power transmitting surfaces exhibiting high wear resistance, and more particularly to endless toothed belts having a wear-resistant fabric cover intimately positioned along the outer surface of the tooth and land portions of the belt, and having a coating adhered to the outer surface of and preferably partially penetrated into the fabric cover, as well as to a method for producing such belts. The coating comprises a wear-resistant composite, which provides improved wear- or abrasion resistance and improved frictional characteristics to the belt, particularly under high load operation thereof.
Endless belts, including V-belts, V-ribbed belts, and flat belting, as well as toothed belts such as synchronous or timing belts and the like, are used in a variety of applications. Examples of power transmission belts, including toothed or synchronous belts, V-belts, and V-ribbed belts are disclosed in U.S. Pat. Nos. 3,138,962; 3,200,180; 4,330,287; and 4,332,576. Examples of methods for producing such belts are disclosed in U.S. Pat. No. 3,200,180 as indicated above and U.S. Pat. Nos. 3,772,929 and 4,066,732. These patent references are merely examples of various types of power transmission belts and state-of-the-art formation techniques thereof.
Toothed belts, generally comprising an elastomeric body portion, an essentially non-extensible reinforcing member and a plurality of driving teeth extending along the underside of the belt at a predetermined pitch, are put to particularly good use in high temperature, high speed and/or high load environments, including various industrial and automotive drive systems. In automotive applications, there is a growing demand for toothed belts which can perform successfully under increasingly high loads and at average operating temperatures of about 120° C. Operating temperature requirements for such applications are expected to reach 150° C. or greater in the near future.
Under such high load, high temperature and/or high-speed conditions, it is common for the teeth of endless toothed belts to deteriorate; the severe shearing stresses on the teeth often result in crack generation and tooth loss. A wear-resistant fabric cover element is used over the tooth and land portions of such belts to shield the elastomeric teeth from such stresses. This modification alone however has not proved completely satisfactory in some particularly demanding applications. Upon extended high load or high-speed operation, such fabric covers tend to wear away, resulting in dimensional changes and/or premature belt failure. Moreover, there is a tendency in such constructions for the underlying belt elastomer to migrate through the weave of the fabric cover during the curing process and/or upon operation, and to thus become exposed at the belt's power transmitting surface. The presence of this relatively high coefficient of friction material at the belt's power transmitting surface results in high noise and frictional heat generation at the belt-sprocket interface upon operation of the belt. Noise generation is viewed as highly undesirable, and frictional heat generation and heat build-up reduce the life of the belt.
One proposed solution to the noise generation and/or frictional heat build up problems common in conventional belt operation has been to reduce the effective coefficient of friction of the power-transmitting surface of the belt. One such approach involves isolating or removing as much of the elastomer as possible from near the surface of the belt where that surface comes in contact with sprocket teeth. Such an approach is taken for example in U.S. Pat. No. 3,772,929, wherein the outer surface of a wear-resistant fabric covering is kept free of belt elastomer, by the presence of a bonded layer of elastomer impervious material adhered to such outer surface.
A second approach has been to incorporate a relatively pure polytetrafluoroethylene (PTFE) layer over the wear-resistant fabric cover element to decrease the effective coefficient of friction of the driving surface of the belt.
A third approach, directed moreover to improving abrasion resistance, has involved coating the motion-transmitting surfaces of a belt with a polymer matrix comprising a fluorine-containing plastic. In European Patent Publication No. 0662571A1, a process for producing the belts is disclosed including the steps of applying such matrix onto the motion-transmitting surfaces of the belt, and then drying the matrix such that it goes through a crosslinking process for bonding itself to the elastomeric belt element.
None of these approaches to the problems of abrasion, noise and/or frictional heat generation in endless belt constructions is believed to be completely satisfactory, particularly in very high load applications. Where the belt surface remains free of belt elastomer by means of a relatively poor abrasion-resistant or low temperature laminate coating on the outer surface of a fabric cover element, high load or high temperature operation generally results in flaking off or melting of the coating. Generally, as a low temperature, low abrasion-resistant laminate flakes or melts off of the fabric layer with continued use, the distance between the center of the load carrying members of the belt and the bottom surface of the land portions between adjacent longitudinally spaced teeth decreases. This dimensional change affects the pitch line diameter of the belt and results in poor tooth-sprocket fit, hence increased belt noise. Moreover, as the coating layer diminishes, the fabric cover becomes exposed to the sprocket, ultimately leading to deterioration of such layer and exposure of the belt elastomer.
A substantially pure PTFE layer incorporated on the surface of a wear-resistant fabric cover element, while producing a reduced coefficient of friction at the driving surface, exhibits very poor wear resistance, and thus would likely wear off of the belt with use, again leaving the wear-resistant fabric layer exposed and presenting the concomitant problems associated therewith, described above.
The incorporation of a coating on a wear-resistant cover element, which comprises a matrix cross-linkable with the belt body and containing a fluorine-containing plastics material, is believed to be similarly inadequate. Application of such coating onto a fabric surface according to the teachings of that disclosure has been found to result in deposition of only a relatively thin layer at the belt surface, which has been found to exhibit inferior wear resistance. It is believed that in the construction proposed in the reference, a relatively large amount of the total fabric cover thickness remains free of the coating material, hence fiber-to-fiber abrasion occurs within the fabric upon operation of the belt. This is believed to lead to a wearing away of portions of the fabric itself, and thus potentially diminished operating life.
Thus, known endless belt constructions or processes for their manufacture have not effectively addressed the combined problems of belt noise, frictional heat generation, and dimensional instability.
Consequently, there remains a need to produce an endless belt, including an endless toothed belt for use in high temperature dynamic applications, which exhibits improved wear resistance, reduced noise during belt operation, which does not exhibit significant frictional heat generation, and which otherwise remains dimensionally stable for an appreciable, predictable operating lifetime.